1.) Field Of The Invention
The present invention relates to a process for cleaning wastewaters from melamine plants and to an apparatus for the same.
2.) Description of the Related Art
Melamine is prepared almost exclusively from urea by the following reaction equation:6H2N—CO—NH2→C3N3(NH2)3+6NH3+3CO2 
The melamine preparation process can be divided into two categories. There are noncatalytic high-pressure processes in which the melamine is prepared in the liquid phase at pressures of >70 bar. In the catalytic low-pressure processes, melamine is synthesized at approximately atmospheric pressure in the gas phase.
What is common to all melamine processes is that the crude melamine from the synthesis stage then has to be purified, since it comprises by-products. To degrade the by-products, the crude melamine is usually worked up in the presence of water, since the by-products go into solution in water under certain conditions. In this way, wastewaters contaminated with nitrogen-containing substances occur at various stages of the melamine plant. These wastewater ingredients are mainly cyclic nitrogen compounds in the form of triazines such as melamine, ureidomelamine, cyanuric acid or oxoamino-triazines (OATs) such as ammeline or ammelide. In addition, depending on the plant status, varying amounts of acyclic nitrogen compounds, for example urea, or else carbonates and sodium or ammonium ions may be present.
The wastewaters contaminated in this way have to be treated to remove their ingredients before they can be discharged.
One example of the wastewater treatment of a melamine process is described by WO 01/46159 A2. According to this, the triazine-contaminated mother liquor obtained in the crystallization of the melamine from aqueous solution is acidified, which crystallizes out the OATs. The OAT suspension thus obtained is subjected to a tangential filtration in which melamine-rich permeate and an OAT suspension as the retentate are obtained. While the melamine-rich permeate is recycled into the plant, the OATs are removed from the retentate. This process has the disadvantage that it is complicated, and the precipitated OATs additionally have to be disposed of.
A further means of wastewater treatment in melamine plants consists in treating the wastewaters in a thermal wastewater treatment plant (TAA), where the triazine-containing wastewater ingredients are hydrolysed under high pressure and high temperature in the liquid phase to CO2 and NH3. Such a process is described, for example, in IT 01282370. There, the crystallization mother liquors of a melamine plant are heated to 180 to 250° C. in a closed vessel under the autogenous pressure of the system and left for 20 to 120 min, as a result of which the melamine and OAT ingredients are degraded.
In a similar manner, according to IT 0128369, triazine-containing melamine wastewater is treated in a closed vessel at a temperature of >250° C. The NH3 and CO2 formed is subsequently stripped off and the resulting pure liquid is recycled into the plant or discharged.
According to DE 102 29 103 A1, the triazine-containing wastewater of a melamine plant is passed in a meandering flow through a heatable apparatus. At temperatures of >190° C. and the system equilibrium pressure, which is about 30 to 60 bar, the wastewater ingredients are decomposed to NH3 and CO2. The reaction takes place in the liquid phase, but small evaporation losses cannot be prevented.
What is common to the processes mentioned is that they are one-stage processes whose hydrolysis apparatus, with regard to pressure, temperature and residence time, is designed for a certain, constant quality and amount of wastewater. In normal operation of the plant, the desired degrees of degradation of the ingredients are thus achieved. However, as soon as operating states occur in which the wastewater varies with regard to concentration and type of ingredients, the required degrees of degradation can no longer be ensured.